The present invention relates to an optically-based frequency synthesizer for generating an electric output signal at a preselected frequency that can be changed over a wide band of frequencies for communication purposes. The present invention is also concerned with a method thereof.
Known in the art, there is the publication entitled "Simple Technique for Improving the Resolution of the Delayed Self-Heterodyne Method" published in Optics Letters/vol. 15, no. 11/Jun. 1, 1990, at p. 640, where FIG. 1 shows a typical delayed self-heterodyne set-up. Note that the laser is not modulated. The output frequency is equal to the input frequency .omega..sub.s. The delay time is also much larger than the coherence time of the laser.
Also known in the art, there is the publication entitled "Linewidth Determination from Self-Heterodyne Measurements with Subcoherence Delay Times" published in IEEE Journal of Quantum Electronics, vol. QE-22, no. 11, November 1986, p.2070-2074. The delay self-heterodyne linewidth measurement technique proposed before this paper normally requires that the delay time be much greater than the coherence time. This paper shows that complex curve fitting can be used for subcoherence analysis. Note that the laser is not modulated in any way. The output beat frequency is simply the input frequency injected by the acoustical-optic modulator. This cannot be used as a frequency synthesizer since the output frequency generated requires an input of the very same frequency.
Also known in the art, there is the publication entitled "Measurement of a Modulated DFB Laser Spectrum Using Gate Delayed Self-Homodyne Technique" published in Electronics Letters, vol. 24, no. 11, 1988 at p.699. In the apparatus shown in FIG. 1 of this publication, the time delay T is set to be much greater than the coherence time to eliminate linewidth reduction and coherence effects. This is seen as problem while the linewidth reduction through the use of a short time delay is seen as a key factor for the success of the present invention. Large modulation factors are used in this paper to generate chirp effects while the present invention will seek to avoid these.
Also known in the art there is the publication entitled "35 GHz Microwave Signal Generation with an Injection-Locked Laser Diode" published in Electronics Letters, Aug. 29, 1985, vol. 21, no. 18, where two modes of a slave laser are excited optically by a master laser. A very narrow linewidth results at the PIN detector at this RF frequency from the beating of the two modes. To function, this operation between the slave modes must be thermally adjusted to match the RF modulation of the master laser. This is a very slow process which eliminates the system for fast hopping frequency synthesizers.
Also known in the art, there is the publication entitled "Miniature Packaged External-Cavity Semiconductor Laser with 50 GHz Continuous Electrical Tuning Range" published in Electronics Letters, vol. 24, no. 16, 1988 at p.988, where an external cavity is used to obtain a narrow linewidth laser output. The adjustment is mechanical, therefore inherently slow. No scheme is set up to get a beat frequency in the RF band.
Also known in the art, there is the U.S. Pat. No. 4,042,891 granted on Aug. 16, 1977 and naming Arnold M. Levine as the inventor. This patent describes a frequency synthesizer or programmable multiple frequency source which uses a voltage-controlled oscillator responsive to an independent control variable and incorporates a fibre optic delay line in an error correcting feedback loop.
Also known in the art, there is the U.S. Pat. No. 4,856,899, granted on Aug. 15, 1989, and naming Hideto Iwaoka et al. as inventors. This patent describes an optical frequency analyzer for measuring an optical frequency spectrum with high accuracy, high resolving power and high stability by heterodyne detecting the incident light with the aid of a local oscillator. This patent does not show the necessary means for generating an electric output signal at a preselected frequency that can be changed over a wide band of frequencies.
Also known in the art, there is the U.S. Pat. No. 4,335,463 granted on Jun. 15, 1982 and naming Joseph Foucard as the inventor. In this patent, there is described a spectrum spread transmission system, particularly adapted for telephone network and using optical fibers. Also, this patent does not show the necessary means for providing an optically-based frequency synthesizer.
The art of frequency synthesis is very old. Many developments have occurred in this field, leading to many types of frequency synthesizers. A good review of these synthesizers, their operating principles and limitations can be found in "Frequency Synthesizers Theory and Design", second edition, written by V. Manassewitsch and published by John-Wiley & Sons in Toronto, 1980. The following reviews briefly the prior art.
In order to compare various methods for frequency synthesis, it is useful to identify parameters of importance when making a comparison.
In a system based on fast hopping frequency synthesis, perhaps to minimize the probability of intercept or the possibility of jamming, it is advantageous to operate over the largest possible hopping bandwidth. To understand this, one can think of a situation where a narrow communication signal could be sent at any frequency over a wide band. To simply try to jam such a signal by blanketing the entire band is prohibitive in terms of power requirements. To try to find the signal and to lock into it and intercept it before the frequency of the signal is "hopped" to a different point is also more difficult, the larger the frequency hopping band. In currently available electronically based systems, the hopping bandwidth available is typically in the order of ten to twenty percent of the operating center frequency.
The availability of frequency bands for communications at "low" frequencies is becoming more and more restrictive. This is part of the reason for the growing need to develop communications systems operating in microwave and millimeterwave frequency bands found in the 17 to 100 GHz range. There are other advantages for operating at higher frequencies such as the fact that higher frequency beams can be made very narrow and directional and thus be more difficult to intercept. In addition, systems operating at higher frequencies generally tend to be more compact, which is a distinct advantage for mobile, battlefield or space requirements. However, operation at higher frequency bands impose difficult manufacturing requirements for synthesizers based on electronic methods. Electronic components must be made to a reduced scale which requires the use of sophisticated and expensive integrated circuits based on MMIC technology (Monolithic Microwave Integrated Circuits) or hybrid technology. Modelling of these conventionally based technologies is difficult if not unreliable. Reliability and reproducibility problems occur, especially with respect to the manufacturing of the components in the foundry and the integration of these components. We also note that conventional methods for "low" frequency operation used for television systems, for example, are totally inadequate. The optical technique in the proposed invention is not limited by such considerations. All the electronics required to govern the optically-based frequency synthesis operate at modest frequencies in the order of the inverse delay time (MHz's) in the delay line of the optical paths. Only the output amplification stage of a synthesizer need be wideband.
Another critical aspect of a fast hopping frequency synthesizer is how fast it can hop from one frequency to the next. This limits the time available for an intercept receiver to find and lock on to the signal. In conventional systems, an important fundamental limit restricts this switching speed. This physical limit is based on the fact that the time for a signal to decay or rise through a filter is inversely proportional to its bandwidth. Filters are required in conventional systems because they are based on mixing and multiplying stages to get to the final desired frequencies. These stages generate unwanted spurs and harmonics which must be filtered out to meet communication requirements. The fastest electronic synthesizers are DDS (discussed below) which have switching speeds in the order of 10 nsec. However, these synthesizers generate high levels of spurious noise and require frequency up-conversion to be used in high frequency wideband communication link.
Phase noise provides a measure of the quality and purity of the signal generated by the frequency synthesizer. The better the phase noise of a synthesizer, the lower will be the bit error rate in communications for a given rate signal power. In addition, the lower the phase noise is, the narrower will be the linewidth of the synthesized signal. This also reduces the power requirements on the synthesized output. The invention proposed makes key use of the coherent properties of lasers to obtain coherent noise cancellation. This noise cancellation provides linewidth reduction in the output to the point that the output can be used for communication purposes. This is a critical aspect of this invention: simply heterodyning two lasers for example results in a totally incoherent interaction between the laser outputs. The resulting linewidth of the output will be typically in the order of 50 to 200 MHz for semiconductor lasers, which is totally inappropriate for most communication purposes.
The volume occupied by a frequency synthesizer, its weight, and power requirements are also important factors for frequency synthesizer. This is especially true for space-bound systems as for mobile platforms, and to a lesser degree, for fixed earth stations.
Another important parameter, especially for a space, or military application, is the number of parts in a complete system. This affects the overall reliability of the system, the cost of assembly and maintenance, and in the case of high frequency electronic systems, increases the difficulties in achieving an acceptable level of reproducibility and yield from production. Most approaches that have sought to introduce optical devices as substitutes for microwave or millimeterwave components have maintained approximately the same overall design configuration and thus have not reduced the total parts count significantly.
A frequency synthesizer is a system that results in the generation of one or more frequencies from one or a few reference sources. The earliest version was probably a crystal controlled oscillator with a bank of crystals switched in manually. Very few frequencies could be generated by such a system. The next improvement to the art was to develop the concept of incoherent synthesis where a number of crystal-controlled oscillators were combined to generate many frequencies with relatively few crystals, Evolving communication requirements led to the development of coherent frequency synthesis. This form of the art provided orders of magnitude improvements in accuracy and stability. Coherent synthesis provides means by which many frequencies are produced from a single reference source. Coherent direct synthesis is limited by the generation of spurious outputs and the large number of stages and parts required to achieve simultaneously a wide bandwidth of frequencies and high frequency resolution. Coherent indirect synthesis is based on phase-lock loop (PLL) principles in generating frequency increments through feedback control. This technique is limited in switching time to the time of the signal in the loop and is limited by the maximum bandwidth over which the loop can maintain frequency lock.
Many synthesizer currently in use are based on coherent direct synthesis. This may be based on analog or direct synthesis. The capabilities of these techniques are limited, however, as new communication requirements push the operating frequencies at higher and higher frequency bands and require faster, lighter, and more energy efficient synthesizers.
Microwave and millimeterwave components, such as mixers and multipliers, used in current synthesizers to achieve "bandwidth spreading", that is an increase in the bandwidth that can be covered by the frequency synthesizer, results in the generation of undesirable frequency spurs. These are normally eliminated by introducing filters at various stages of the frequency synthesis. If the filters are not in place, unsatisfactory system performance is likely. However, the presence of these filters imposes fundamental limits on switching speeds between frequencies which can be explained by the uncertainty principle. As an estimate, the time for a signal to propagate through a filter is approximately the inverse of its bandwidth, which puts limits on switching speeds. In addition, each mixer/filter or multiplier/filter stage introduces system losses which must be compensated by higher power inputs.
Much effort is being placed on GaAs MMIC technology to develop the compact and intricate circuitry required to apply coherent direct frequency synthesis to systems in the EHF frequency band. Important manufacturing problems must be resolved to make these techniques economically viable. In addition, filters are not easily integrated on active device chips based on GaAs. This leads to numerous interconnections and reproducibility problems with respect to system performance. Skilled technicians are therefore normally required to assemble, test and adjust these circuits leading to high fabrication and repair costs.
A recent advance in the art has been the development of Direct Digital Synthesis (DDS) which is based on the reconstruction of an output frequency from digitized look-up tables. This type of synthesizer has suffered principally from spurious outputs due to bit limitations for fast A/D convertors although clever means can reduce the level of these significantly. The other fundamental limitation of DDS is that the natural output of the synthesizer is limited to approximately half its operating clock frequency. This can be explained by the Nyquist sampling theorem. Present integrated circuit technology on which DDS depends is limited to operating clock frequencies in the order of 1 GHz. Thus to achieve a wide hopping spectrum based on DDS, a frequency expander section consisting of multipliers must be added to the unit which introduces limitations on switching speed, signal purity, part count and other factors as discussed earlier.
Another development that can be found in the prior art is the use of hybrid techniques which consist in combining two types of frequency synthesizers to form a complete system which hopefully combines the best of each type while minimizing their individual limitations. Indeed, the present invention could also be incorporated with other synthesizers. An example of such a system might be the combination of a DDS capable of providing excellent frequency resolution with a direct analog synthesizer capable of supplying only ten to one hundred frequencies separated by the frequency span covered by the DDS. In this case, the DDS provides the fine resolution while the direct analog synthesizer provides the wide bandwidth portion. Note that this requires additional complexity when compared to a single synthesizer and the system is limited by electronic considerations in bandwidth and switching speeds.
One of the simplest methods to introduce optical techniques to the art of frequency synthesis is to simply beat two lasers together. Essentially, the technique requires that the two lasers be tuned to frequency outputs very close together, usually by individual thermal adjustments to the lasers, although other techniques are possible. Their outputs are then beat against each other and the difference frequency is converted into an electrical signal. For the signal to be useable in a communications system, it is required either that the lasers have extremely narrow linewidths or that they somehow be phase-correlated. The former case requires expensive, highly stable lasers who are not usually compact. These lasers usually have a limited switching speed and limited frequency tunability. The latter requires that the lasers be phase locked, perhaps through a master-slave arrangement with a third laser. This type of technique has only been achieved over a relatively narrow bandwidth.
One might consider using external cavities with semiconductor lasers to narrow their linewidth but fundamental limits arise to the linewidth narrowing that can be achieved. In addition, the switching speed of the laser would be related to the external cavity return time, and hence the switching speed of the laser can be made too slow to use in a hopping system.
Finally, another consideration is that temperature and current gradients can affect each laser separately, which may result in an unstable system.
Yet another use of optical techniques found in the prior art is the use of optical components to replace microwave and millimeterwave components in the frequency expansion section of the frequency synthesizer. This type of utilization does not, however, result in important changes in the overall approach and virtually the same part count as in a completely electronic synthesizer results. In addition, optical components have a very limited efficiency as microwave component substitutes which may result in higher power requirements.